This invention relates, in general, to doping silicon substrates and more particularly, to compensating for the affects high concentrations of dopant have on substrates.
It is well known in the art that the performance of a semiconductor device can be improved by employing epitaxial films formed on substrates with low resistivity. The resistivity of a substrate crystal can be reduced by doping the substrate with donor or acceptor atoms which will make the substrate either n type conductivity or p type conductivity. Typical dopant atoms used, however, do not have the same atomic radius as the substrate crystal atoms. Silicon atoms have an atomic radius of 1.18 .ANG. and common dopant atoms such as phosphorus and boron have an atomic radius of 1.08 .ANG. and 0.86 .ANG. respectively.
The size mismatch between the dopant atoms and the silicon substrate atoms will induce strain in the crystal. This strain is a result of the substrate lattice contracting as the lattice is forced to compensate for the high concentration of smaller dopant atoms. As the concentration of these dopant atoms increases, the lattice will continue to contract which will further reduce the lattice constant. Current commercial processes do not distribute the dopant uniformly. The radial and axial non-uniform dopant distribution will generate internal stress. This stress will be replicated and amplified by an overlying epitaxial layer and as the thickness of the epitaxial layer increases so too will the strain in the layer.
A second difficulty with forming epitaxial layers on a substrate arises from misfit dislocations at the substrate interface. In growing a lightly doped phosphorus n- epitaxial layer on top of a heavily doped phosphorus n+ substrate, the lattice parameter of the n- layer is larger than the lattice parameter of the n+ substrate. Therefore, a layer of misfit dislocations is generated between the n- and n+ layers. These dislocations will hinder the performance of a semiconductor device as the dislocations contribute to junction leakage
A high dopant concentration will also pose problems due to the presence of a diffusion gradient. The high concentration of dopant in the substrate will naturally want to diffuse to areas of lower concentration such as to the overlying epitaxial layer with a lower dopant concentration This is likely to drive dopant atoms into the epitaxial layer or auto-dope subsequent layers formed over the substrate.
Accordingly, in order to form n type substrates with resistivities below 0.01 ohm-cm it is necessary to dope a substrate with phosphorus concentrations above 4.5.times.10.sup.18 atoms per cm.sup.3. A method is necessary to compensate for the lattice strain created by dopant atoms and retard dopant atoms from diffusing into layers overlying the substrate.